1. Field of the Invention
The invention relates to an apparatus and method for the delivery of an inspired gas (e.g., supplemental oxygen (O2) gas) to a person combined with sampling of the gas exhaled by the person, such sampling for use, for example, in monitoring the ventilation of the person or for inferring the concentration of a drug or gas in the person's blood stream. More particularly, the invention relates to an apparatus and method where such delivery of the inspired gas and gas sampling are accomplished without the use of a sealed face mask.
2. Description of Related Art
In various medical procedures and treatments performed on patients, there is a need to deliver a desired inspired gas composition, e.g., supplemental oxygen, to the patient. In procedures involving the delivery of anesthesia or where a patient is otherwise unconscious and ventilated, the delivery of oxygen and gaseous or vaporized or nebulized drugs is typically accomplished via a mask that fits over the patient's nose and mouth and is sealed thereto or by a tracheal tube. In other procedures, however, for example, where a patient may be sedated, but conscious and breathing on their own, the delivery of supplemental oxygen or inspired gas may be accomplished via a mask or by nasal cannulae (tubes placed up each nare of a patient's nose), connected to a supply of oxygen or the desired gas composition.
Taking oxygen as one example of an inspired gas to be delivered to a person, the primary goal of oxygen supplementation (whether mask-free or otherwise) is to enrich the oxygen concentration of the alveolar gas, namely, the mixture of gas in the alveoli (microscopically tiny clusters of air-filled sacs) in the lungs. In a person with normal lung function, the level of oxygen in the deepest portion of the alveolar sacs is essentially reflected at the end of each “tidal volume” of exhaled gas (the volume of gas in one complete exhalation). The gas sample measured at the end of a person's exhalation is called the “end-tidal” gas sample.
So, for example, if a person breathes room air, room air contains 21% oxygen. When the person exhales, the end tidal gas will have about 15% oxygen; the capillary blood has thus removed 6% of the oxygen from the inhaled gas in the alveoli, to be burned by the body in the process of metabolism. Again, a simple goal of any form of oxygen supplementation is to increase the concentration of oxygen in the alveolar sacs. A convenient method of directly measuring or sampling the gas in alveolar sacs is by continuously sampling the exhaled gas at the mouth or nose and identifying the concentration of oxygen at the end-tidal point, a value that is reasonably reflective of the oxygen concentration in the alveolar sacs. Thus, one can compare the effectiveness of oxygen delivery systems by the amount that they increase the end tidal oxygen concentration.
If a person breathes through a sealing face mask attached to one-way valves and inhales a supply of 100% oxygen, the end tidal concentration of oxygen goes up to 90%. More specifically, once inert nitrogen gas has been eliminated from the lungs (after pure oxygen has been breathed for several minutes), alveolar gas will contain about 4% water vapor and 5% carbon dioxide. The remainder (about 90%) will be oxygen. Thus, the best oxygen delivery systems typically increase end tidal oxygen from a baseline of 15%, when breathing non-supplemented room air, to 90% when breathing pure oxygen. Although sealed face-masks are relatively effective oxygen delivery systems, conscious patients, even when sedated, often find masks significantly uncomfortable; masks inhibit the ability of a patient to speak and cause anxiety in some patients.
Nasal cannulae, on the other hand, do not typically cause the level of discomfort or anxiety in conscious patients that masks do, and thus, from a patient comfort standpoint, are preferable over masks for the delivery of oxygen to conscious patients. Nasal cannulae are, however, significantly less effective oxygen delivery systems than sealed face masks. Nasal cannulae generally increase the end tidal oxygen concentration to about 40% (as compared to 90% for a sealed mask). Nasal cannulae are less effective for at least two reasons.
First, when a person inhales, they frequently breathe through both nasal passages and the mouth (three orifices). Thus, the weighed average concentration of inhaled oxygen is substantially diluted to the extent of mouth breathing because 21% times the volume of air breathed through the mouth “weights down the weighted average.”
Second, even if a person breathes only through their nose, the rate of inhalation significantly exceeds the supply rate of the nasal cannula (typically 2–5 liters/min.) so the person still dilutes the inhaled oxygen with a supply of 21% O2 room air. If the nasal cannula is flowing at 2 liters per minute and a person is inhaling a liter of air over 2 seconds, the inhalation rate is 30 liters per minute, and thus, most of the inhaled volume is not coming from the nasal cannula, but rather from the room. Increasing the oxygen flow rate does not effectively solve this problem. First, patients generally find increased flow very uncomfortable. Second, increased inspired gas flow dilutes (washes away) exhaled gases like carbon dioxide and/or exhaled vapors of intravenous anesthetics or other drugs. When this happens carbon dioxide cannot be accurately sampled as a measure of respiratory sufficiency. Also, a drug such as an inhalational or intravenous anesthetic, cannot be accurately sampled as a measure of the arterial concentration of the drug from which, for example, the level of sedation might be inferred. There is a need in various medical procedures and treatments to monitor patient physiological conditions such as patient ventilation (the movement of gas into and out of the lungs, typically measured as a volume of gas per minute). If the patient does not move air into and out of the lungs then the patient will develop oxygen deficiency (hypoxia), which if severe and progressive is a lethal condition. Noninvasive monitoring of hypoxia is now available via pulse oximetry. However, pulse oximetry may be late to diagnose an impending problem because once the condition of low blood oxygen is detected, the problem already exists. Hypoventilation is frequently the cause of hypoxemia. When this is the case, hypoventilation can precede hypoxemia by several minutes. A good monitor of ventilation should be able to keep a patient “out of trouble” (if the condition of hypoventilation is diagnosed early and corrected) whereas a pulse oximeter often only diagnoses that a patient is now “in” trouble. This pulse oximetry delay compared to ventilatory monitoring is especially important in acute settings where respiratory depressant drugs are administered to the patient, as is usually the case during painful procedures performed under conscious sedation.
Ventilatory monitoring is typically measured in terms of the total volumetric flow into and out of a patient's lungs. One method of effective ventilatory monitoring is to count respiratory rate and then to measure one of the primary effects of ventilation (removing carbon dioxide from the body). Certain methods of monitoring ventilation measure the “effect” of ventilation (pressure oscillations, gas flow, breath sound and exhaled humidity, heat or CO2 at the airway). Other ventilation methods measure the “effort” of ventilation (e.g., transthoracic impedance plethysmography, chest belts, respiratory rate extraction from optoplethysmograms). Effort-based ventilation monitors may be less desirable because they may fail to detect a blocked airway where the patient generates the effort (chest expansion, shifts in blood volume, etc.) but does not achieve the desired effects that accompany gas exchange.
There are a variety of ventilation monitors such as 1) airway flowmeters and 2) capnometers (carbon dioxide analyzers). These monitors are used routinely for patients undergoing general anesthesia. These types of monitors work well when the patient's airway is “closed” in an airway system such as when the patient has a sealing face mask or has the airway sealed with a tracheal tube placed into the lungs. However, these systems work less well with an “open” airway such as when nasal cannulae are applied for oxygen supplementation. Thus, when a patient has a non-sealed airway, the options for tidal volume monitoring are limited. With an open airway, there have been attempts to monitor ventilation using capnometry, impedance plethysmography, humidity, heat, sound and respiratory rate derived from the pulse oximeter's plethysmogram. Some of the limitations are discussed below.
Nasal capnometry is the technique of placing a sampling tube into one of the nostrils and continuously analyzing the carbon dioxide content present in the gas stream thereof. Nasal capnometry is relatively effective provided that 1) the patient always breathes through his/her nose, and 2) nasal oxygen is not applied. More specifically, if the patient is talking, most of the exhalation is via the mouth, and frequent false positive alarms sound because the capnometer interprets the absence of carbon dioxide in the nose as apnea, when in fact, it is merely evidence of talking. Some devices in the prior art have tried to overcome this problem by: manual control of sampling from the nose or mouth (Nazorcap); supplementing oxygen outside of the nose while sampling for CO2 up inside the nose (BCI); providing oxygen in the nose while sampling CO2 from the mouth (BCI); and supplying oxygen up one nostril and sampling for CO2 up inside the other nostril (Salter Labs). None of these already-existing systems provide oxygen to both the nose and mouth or allow automatic control of sampling from either site or account for the possibility that one nostril may be completely or partially obstructed compared to the other one. Further, if nasal oxygen is applied to the patient, the carbon dioxide in each exhalation can be diluted significantly by the oxygen supply. In this case, the capnometer may interpret the diluted CO2 sample as apnea (stoppage in breathing), resulting once again, in frequent false positive alarms. Dilution of CO2 may also mask hypoventilation (detected by high CO2) by making a high CO2 value appear artifactually normal and thus lull the clinician into a false sense of security, that all is well with the patient.
Impedance plethysmography and plethysmogram respiratory rate counting also suffer drawbacks as primary respiratory monitors. Both devices measure the “effort” of the patient (chest expansion, shifts in blood volume). Impedance plethysmography is done via the application of a small voltage across two ECG electrode pads placed on each side of the thoracic cage. In theory, each respiration could be detected as the phasic change of thoracic impedance. Unfortunately, the resulting signal often has too much noise/artifact which can adversely affect reliability. Respiratory rate derived from the pulse oximeter's plethysmogram may not diagnose apnea and distinguish it from complete airway obstruction, thus misdiagnosing apnea as a normal condition (a false negative alarm state).
The arterial concentration of an inhalational or intravenous drug or gas is clinically useful and may be inferred from the end-tidal concentration of the drug or gas measured in the gases exhaled by the patient. The end-tidal concentration of a desired component of the exhaled gas mixture can be monitored and used to infer the arterial concentration. Examples of drugs and gases that can be monitored include, among other things: propofol, xenon, intravenous anesthetics and sedatives, and water vapor.
Various inspired gas compositions may be administered to patients for different purposes. Oxygen diluted with air may be used instead of pure O2 to reduce the risk of an oxygen-enriched micro-environment that may support or promote ignition of a fire, especially for those procedures using lasers (such as laser resurfacing of the face). An oxygen-helium mixture may be used to reduce the resistance to flow. An oxygen/air/bronchodilator mixture may be used to treat bronchoconstriction, bronchospasm or chronic obstructive pulmonary disease (COPD). A mixture of O2 and water vapor may be used to humidify and loosen pulmonary secretions.
In view of the above drawbacks to current systems for delivering inspired gas and gas sampling, including monitoring ventilation, there is a need for an improved combined system to accomplish these functions.